After all, although the country's average renewable generation for the month of March exceeded 103 percent of consumption, per Quartz, it still relied on some fossil fuel and imported electricity since water-and-wind-generated power is so unpredictable. Still, Portugal is confident that its mainland electricity needs will be completely satisfied by renewables by 2040.

Other countries like Denmark and Germany are also pioneers in the usage of renewable energy. Meanwhile Sweden is committed to reaching net-zero greenhouse gas emissions by 2045, according to the new Climate Act.

Only the future can tell whether these countries goals are even attainable, but there are current challenges to overcome first, like the short periods the grid in Portugal could run, which included two 70-hour spans. And while renewable energy may be unpredictable, Mother Nature is even more so, which creates special challenges in sustaining renewables for multiple years. For example, per Euractiv, last year renewables only accounted for 6 percent of Portugal's electricity because of a drought that reduced its hydro capacity. According to Popular Science, one solution going forward might be to fill batteries with influxes of renewable energy, and then use those batteries to ride out periods when renewable energy generation is limited.